The reduction of lead alkyls in petrols has caused refiners for many years now to envisage incorporating different compounds and in particular alcohols and esters into petrol, to permit an increase in the octane number. Besides methanol which is one of the most attractive known additives, MTBE (methyl-tertiobutyl ether) has anti-knock properties which permit an improvement in the quality of the petrols and an increase in their octane number, such increase being greater than that which is obtained with methanol. MTBE also has many other advantages such as:
a boiling point corresponding to that of the components of the petrol which have the lowest anti-knock properties, PA0 a vapor pressure which is compatible with the above-mentioned components, PA0 an excellent freezing point, PA0 low solubility in water, PA0 complete miscibility with hydrocarbons, etc.
MTBE is generally produced from isobutene and methanol in accordance with the following reaction: ##STR1##
Isobutene is generally contained in C.sub.3 -C.sub.4 olefinic cuts obtained from the effluents from catalytic cracking, steam cracking, thermal cracking and visbreaking. However the amounts of isobutene provided by those different processes are not sufficient to permit a broad development of the MTBE production process.
It is for that reason that, in order to produce larger amounts of isobutene, it has been proposed that the butenes contained in the effluents from the above-mentioned processes are to be completely or almost completely isomerized to provide isobutenes.
Many processes associated with many catalysts have been proposed in the literature. The catalysts used are generally based on alumina or more particularly aluminas which have been activated or treated with vapor (U.S. Pat. No. 3,558,733), whether involving alumina h or g, halogenated aluminas (U.S. Pat. No 2,417,647) bauxite, aluminas treated with derivatives of boron, silicon (U.S. Pat. No. 4,013,590, U.S. Pat. No 4,038,337 GB 2 129 701 and U.S. Pat. No 4,434,315) or zirconium and various silica-aluminas, etc.
Most of those catalysts exhibit a relatively low level of conversion per pass and a low degree of selectivity due to the parasitic reactions such as cracking and polymerization, which latter also give rise to a rapid drop-off in the levels of performance.